1. Field of the Invention
The present invention relates generally to a gas detecting system for detecting and annunciating the presence and location of select gases at a plurality of locations, and more particularly pertains to a system for detecting and annunciating the presence and locations of accumulations of combustible vapor and air mixtures prior to their reaching a predefined hazardous level. The subject invention was designed in the context of a combustible vapor detection system for an aircraft, although the teachings herein are of a much broader nature. For instance, embodiments of the present invention might have direct suitability for detecting a combustible mixture of gas in a landfill or a mining environment wherein methane accumulations occasionally are known to present hazardous conditions. Also, many of the teachings herein, particularly those on the diagnostic features of the present invention, have direct application in gas detecting systems other than those designed particularly for the detection of combustible vapors.
2. Description of the Prior Art
Betz U.S. Pat. No. 3,997,837 discloses a gas analyzer system for monitoring the gaseous products of a process such as the exhaust gases from the combustion process of an oil burner. In Betz the electrical output of a semiconductor measuring cell is coupled to the noninverting or positive input of a differential amplifier, while the negative input thereof is coupled to a preset threshold voltage. When the positive output from the measuring cell exceeds the threshold voltage, the amplifier actuates several relays which control, for example, regulation and alarm devices, and an indicating element such as an LED annunciator. However, Betz is designed for a different type of application than the present invention, and suggests, for example, that a plurality of measuring locations can be monitored by automatically and sequentially connecting a plurality of measuring sensors to one differential amplifier. This reference also suggests that it is often appropriate to combine the signals from a plurality of measuring sensors to form a common output signal. The processing of a plurality of the sensor output signals in a manner as suggested by Betz would be clearly inappropriate to the type of combustible vapor detection system of the present invention.
Gas sensing elements constructed from a semiconductive material such as tin oxide which changes its resistance/conductance in dependence upon its surface adsorption of select types of gases have been developed by the Figaro Engineering Corporation of Osaka, Japan, and are disclosed in different stages of development in Taguchi U.S. Pat. No. 3,625,756, Taguchi U.S. Pat. No. 3,695,848, and Taguchi U.S. Pat. No. 3,900,815. These types of gas sensors are quite well known in the prior art as TGS (Taguchi Gas Sensor) sensors. TGS sensors are generally heated by an associated heating element, and are the type of sensor utilized in one designed and operable embodiment of the present invention. The types of sensors offered by the prior art, however, were not originally designed for an aerospace application as concerns the present invention. However, TGS sensors can be adapted to different applications by selectively doping them to improve their sensitivity to given types of gases.
Klass U.S. Pat. No. 3,864,628 is also somewhat pertinent to the present invention by disclosing TGS sensors utilized in combination with selective gaseous permeable membranes. This reference also discusses alarm techniques and annunciation set levels which can be utilized with a TGS sensor.